Scanning oscillator



Nov. 3, 1936. P. T. FARNSWORTH SCANNING OSCILLATOR Filed April 3, 1935 2 Sheets-Sheet 1 INVENTORY ATTORNEY PH/LO T. FARNS WORTH.

Nov; 3, 1936; P. T. FARNSWORTH SCANNING OSCILLATOR 2 Sheets-Sheet 2 Filed April 3, 1935 PRIMARY CURRENT.

VOL TS 0N GRID.

INVENTOR, PI-l/LO 7: FARNSWORTH.

ATTORNEY Patented Nov. 3, 1936 SCANNING OSCILLATOR Philo T. Farnsworth, San Francisco, Calm, assignor 'to Farnsworth Television Incorporated,

a corporation of California Application April 3, 1933, Serial No. 864,180

6 Claims.

This invention relates to oscillators for producing non-sinusoidal waves of predetermined form, and particularly to oscillators for producing waves having a portion of substantially constant slope,

for producing scanning fields in cathode ray television equipment. This application is a continuation in part of my copending application, Serial No. 550,654, filed July 14, 1931.

Among the objects of this invention are: to

. J provide a type of oscillator suitable for producing either the high frequency or the low frequency scanning field for cathode ray television equipment, depending upon the electrical constants of the design; to provide an oscillator for producing 5 non-sinusoidal or saw-toot waveforms which are not dependent upon ionization phenomena; to provide an oscillator for television use having stable and predeterminable characteristics; to provide an oscillator which will fall into step readily with a synchronizing pulse; to provide an oscillator wherein the natural curvature of the waveform may be corrected to produce a wave characterized by a substantially straight line form over a major portion of its cycle; and to provide an oscillator whose frequency characteristics are substantially independent of resonance, and whose waveform is controllable, externally of the tube, within wide limits.

Other objects of my invention will be apparent or will be specifically pointed out in the descrip- 3 5 Referring to the drawings:

Figure 1 is a circuit diagram illustrating the oscillator of my invention as applied to the supply of high frequency scanning current for a television receiving system.

40 Figure 2 comprises graph showing the waveform of plate current and grid potential in one embodiment of the invention.

Figure 3 is a circuit diagram of the oscillator as utilized to supply low frequency scanning fields.

45 Considering the invention broadly, it comprises a vacuum tube of the hot-cathode type, preferably provided with an additional grid or control electrode through which synchronizing impulses are applied to the oscillator. Included in the plate 50 circuit of the tube is the primary of a transformer,

-- cation constant the ratio may be unity or less, but

in any case the primary and secondary circuits should be non-resonant. The secondary is connected to the grid of the tube and is so poled that an increase in plate current will drive the grid strongly positive. Biasing for the grid is preferably provided by a relatively large condenser bridged by an adjustable grid leak.

Coupled into the plate circuit, either directly in series, or through the same transformer as is used for coupling grid and plate, is an impedance 10 element which may be the useful work circuit of the tube, or may be a corrective element provided for the sole purpose of controlling the waveform.

As will be explained below, where the object of the oscillator is to produce saw-tooth current waves 15 having a substantially linear increase in current during a major portion of the cycle, this corrective element takes the form of an iron core inductance, wherein the core exhibits saturation phenomena within the range of the current flow through the tube. Where other types of waveform are desired this corrective element may, of course, be given other characteristics. The usual plate supply is, of course, provided.

The fundamental characteristics of this oscil- 25 lator arise through the coupling between grid and plate. The high step-up ratio of the transformer causes the tube to dump or flop. That is, the tube acts almost purely as a switch. When a voltage is first applied to the plate, and our- 30 rent starts to flow. a high positive potential appearson the control grid, causing the tube to exhibit minimum impedance. Under these conditions, the current flow in the plate circuit is conditioned primarily by the impedance external to the tube, or at least external to the plate circuit, and thegrid circuit itself acts as a load on the plate circuit which reduces its impedance for so long a time as the grid continues to draw current. when saturation in the tube or any other cause operates to decrease the amount of plate current, the grid promptly swings negative, causing a further decrease in plate current, which again reflects into the grid circuit, so that the flow is interrupted-practically instantly. This reestablishes open circuit conditions in the plate circuit, after which the cycle repeats.

It is obvious that the performance of this device wlll be greatly modified by the conditions within the load and corrective circuits, but before considering this in detailtwo embodiments of the invention will be described, and the efiects producible will be taken up as afiected by the conditions obtaining in these embodiments.

The embodiment shown in Figure 1 illustrates 5 the application of the invention as used to supply high frequency scanning. potentials in a television receiving system, as is described in my copending application, Serial No. 550,654, above mentioned. 5 A television signal, comprising icture impulses interspersed with scanning pulses is supplied """through the'leads l and 2, the lead I being connected through a grid condenser 3 to the control electrode 5 of an oscillight or cathode ray television receivertube 5, while the lead 2 connects to the cathode I of the tube. The perforated anode 8 accelerates electrons from the cathode to form a cathode ray beam which impinges upon a fluorescent screen'formed on the face ill of the tube. The grid 5 is biased by a battery or'other source ii through a grid resistor or leak l2.

The beam is deflected by a magnetic field produced by adefiecting coil l3, and the oscillator which is the subject of this invention supplies the zaicurrentiogthis coil through the medium of a winding l5 which is one of several coupled by a ferro-magnetic transformer core l6.

The primary coil I 'l of the transformer has one end connected to the plate l8 of thevacuum tube 53 20, the other-end of the winding being connected ""iir'series with a suitable plate supp y 2|, and

thence to ground. A secondary winding 22, hav- W ing a material step-npiratio as compared with the primary l1, connects through a relatively large 30 .gridcondenser 23, bridged by an adjustable leak 35 center tap on a winding 21 of a filament transformer, the winding being linked with the core 28 and feeding the cathode 30 of the vacuum tube 20. The tube is preferably supplied with an auxiliary' grid 3i which connects through a blocking condenser 32 and resistor 33 with the grid of the oscillight. A positive bias is applied to the aux- ""iliary grid, through a resistor 34, by a battery or other source 35 whose negative end also connects 5 to ground. I

As is described in my copending application above identified,- a portion of the output of the oscillator is preferably utilized to supply anode potential for the oscillight. An additional wind- 50 ing 36 on the coupling transformer connects through a condenser 31 to'the grounded plate of a diode 38. The cathode of this diode is supplied by a winding 82 on the filament transformer, the high potential winding 36 connecting to a mid tap 55 on the winding 62. The filament transformer is provided with a primary 39, connected to the ordinary alternating current supply leads 4 I. The anode circuit of the 'osciilight may therefore be traced from the high potential side of the winding 60 36, through a lead 40 connecting with the anode 80f the oscillight, from the oscillight filament to ground, and thence from the ground back through the diode 38 to the low potential side of the wind- -ing 36. The function of the diode is, of course, 5 to rectify this current, while the condenser 37 functions as. a filter under the conditions described. This portion of the equipment is fullydescribed and claimed in the copending application previously referred to.

70 The embodiment of the invention shown in Figure 3 is that preferred for supplying the low frequency scanning current. In this. diagram the-filament circuits have been omitted, since they are essentially similar to those shown in the 75 first figure and the resultant simplification in .circuit. During this initial portion of the cycle the drawing increases its clarity. In this figure. the tube 50 is similar to the tube 20 of the first figure, having a plate 5|, cathode 52, grid 54, and auxiliary synchronizing grid 55.

The plate 5| connects to the primary 56 of 6 a step-up transformer 51, whose secondary 59 is factory for this purpose. a

The plate current is supplied to the tube through a resistor 64, across which there is bridged a scanning circuit comprising the coils 65 in series with a blocking condenser 66 of about ten microfarads capacity. In practice the resistor 64 has a value of about 15,000 ohms, While that of the scanning coils is approximately 8,000 ohms. A resistor 61, preferably of considerably higher value than that of resistor 64 is bridged across the coils, this re'sistor being just low enough in. value to prevent impact oscillations being set up in the'coils during the steeper slope portion of the current wave. The circuit is supplied from a source 69 whose negative end is grounded.

In order to prevent undue stress on the insulation of the transformer and on the'tube it is usually desirable to connect a resistor 10 in series with a condenser H, from the plate 5| to ground. This circuit is in the nature of a safety valve or relief by-pass, and its constants depend upon the maximum potential which may safely be permitted on the plate of the tube.

A synchronizing pulse may be applied to the tube through the lead 12, which is shown as connected to ground to a grid leak 14."

We may now consider the detailed operation of the invention in connection with Figure 3. Starting-with. the cathode excited, as the plate circuit is first closed, a current will start to fiow in this circuit, the potentials provided by the battery being divided across the scanning coil circuit, the inductor 62, primary 56, and the tube. In the ordinary oscillator at least half and usually more of'the potential drop will occur in the tube, but in the present instance, owing to the high step-up ratio of the transformer, the grid 56 at once swings so strongly positive that the tube itself becomes of minimum impedance, and the major voltage drop occurs in the external the grid may be much more strongly positive than the plate, and the major current flow from the cathode may actually be to the grid instead of the plate. This imposes a relatively large load on the transformer, decreasing its efl'ective primary impedance, with the result that the major voltage drop in the plate circuit occurs across the inductance 62, whose voltage drop exceeds all the rest of the circuit elements put together.

The inductance permits a gradually increasing current to flow, but this current at once begins to saturate the core, which decreases the effective reactanc'e in the circuit and consequently. permits a constantly increasing voltage to ap- 75 pear on the plate. In thehieantime the condenser 60 has started to charge due to the flow of grid current, and during the ensuing portion of the cycle the plate continues to grow more positive and the grid less positive, 50 that the filament current isgradually transferred from the grid to the plate. Were the inductance 62 an air core device, so that the inductance remained at substantially constant value, the rate of increase in the plate current would gradually fall off, but owing to the saturation efiect in the inductance and the decreased effective plate resistance of the tube due to the higher voltage applied thereto, this condition is corrected or compensated, and the period of increasing current is characterized by a substantially constant rate of change'as is shown by the portion 15 of the upper graph of Figure 2.

It will be noted that throughout this period there will also be a substantially constant rate of increase in the current in the scanning coils.

Although thesecoils represent ,a large number of turns, their air core construction and the relatively lowrate of change of the current therein, renders them a primarily resistive load. At a scanning frequency of say 24 to 25 cycles, the inductive component of these coils cannot be relied upon to control the'waveform.

The end of this increasing portion in the cycle occurs when saturation current effects begin to manifest themselves within the tube. There then arrives the transition point when the plate current ceases to increase, The positive charge imposed upon the grid by the transformer thereupon disappears, and the negative charge which has been accumulated bythe condenser 60 can take its full efiect, causing an actual decrease in plate current. This causes a further negative charge to be imposed upon the grid 54 by the transformer, so that the tube immediately dumps", a tremendously high negative potential pulse appearing on the grid as indicated by the portion 16 of the lower curve of Figure 2. The plate current is blocked completely, although it does not fall off instantly owing to the tremendous voltage pulse tending toward .its continuation which appearsin the plate circuit due to its high inductance. It is' the function of the resistor Ill and condenser 1| to limit the value of this pulse. The decrease in plate current takes the form shown by the portion 11 of the upper graph. The discharge of the condenser 60 through the transformer causes the slight negative pulse 19 in the primary current, and the cycle repeats when this current again reaches zero; The grid voltage throughout the increasing portion of the cycle" is shown by the positive portion Bll of the grid voltage curve, this voltage rising to a maxi mum and then falling off when the condenser 50 charges and discharges.

It is to be noted that in this oscillator the grid circuit exercises a double function. That is, it not only serves to apply the usual control voltages to the grid, thus controlling the impedance of the plate circuit internally of the tube, but it also imposes a load upon the plate circuit which causes impedance changes in that circuit externally of the tube. The grid; condenser, moreover, adds to. its usual functionof producing a- It follows from these facts that there are several possible methods of description of the operation. all equally true, but differing primarily in em phasis. The one adopted here is merely that in which the emphasis is directed most strongly toward the factors which determine the design of equipment for producing a predetermined waveform, i. e., the corrective reactance of. the primary circuit and the phase or load control in the grid circuit.

The elements present in the circuit of Figure 3 are also present in the circuit of Figure 1, but in this case, instead of the various elements of the circuit being included in series with the transformer primary, their effect is introduced into that circuit through the transformer coupling. The controlling inductance, which limits inductive effect of the coils themselves may well be ample to control the rate of change in the primary current. The grid condenser 23 and grid leak 24 have the same efiect as the grid condenser 60 and grid leak 6! of Figure 3, although they occupy a different position in the circuit. The function of the resistor 10 and condenser H is filled by the winding 36 and its associated circuits, comprising the diode 38 and the space circuit of the cathode ray tube 6.

The natural frequency of each of the two forms of oscillator is controlled by adjusting the value of the grid leak, thus determining the phase of the grid circuit load and the time required for the grid condenser to charge to a sufliciently high value to start the "dumping action of the tube. Irrespective of this setting, however, the appearance of a negative pulse of suificiently high value on the auxiliary or synchronizing grid will start the action, holding the oscillator in step even though the frequency of the synchronizing pulses may depart very materially from the natural frequency setting of the device. The frequency may also be controlled by regulating the voltage of the plate supply.

It will be obvious that by changing either the grid circuit impedance, the corrective impedance, or both, entirely different waveforms could be generated in-an oscillator of this type. Resistance or capacity in the plate circuit will completely alter the rate of change in the plate current, while an inductive load in the grid circuit would cause the grid voltage to rise as the cycle progressed instead of fall. It would appear unnecessary to go into the various modifications possible with the combination described, since the effect of various types of impedance elements is well known.

Both forms of waveform control are utilized in each of the modifications of the oscillator here shown. In the low frequency type of oscillator the plate circuit form of correction is utilized more fully than in the high frequency type, since the requirements are somewhat less rigorous on the higher frequency equipment, and it is easier to utilize a smaller portion of the saturation current of the tube, thus avoiding the tendency toward curvature in the waveform, than it is to carry the increasing current portion of the cycle toward the limit and correct for denser at the. fundamental frequency.

the curvature. In the high frequency type of oscillator the impedance of the grid condenser to the'fundamental frequency should .be low, a condenser-of approximately microfarad being suitable for frequencies of two to three thousand cycles, the grid leak having an impedance about one order of magnitude greater than the con- At the frequencies used in the formsshown in Figure 3 this would require a 10 microfarad grid condenser, which would be unduly bulky. A smaller grid condenser is therefore used,with a grid leak of such value as to give the required time constant, and the distortion which would otherwise ensue from this departure from optimum values is compensated for by thesaturation efiect in the corrective impedance.

Perhaps the greatest advantage of the type of oscillator here described is the fact that it provides the sudden break-down characteristics which are usually found in gas conduction tubes with the stability and reproducibility of high vacuum tube phenomena. Very satisfactory saw-tooth waves may be produced with tubes of the glow discharge type when proper conditions of pressure and temperature obtain, but these factors vary so much with use and even with surrounding temperature that it is extremely diflicult to obtain consistent results therewith. The oscillator of the present invention, however, will operate consistently for long periods, and is not afiected materially by change of tubes, and it is in this, almost as much as in its wide flexibility of waveform, that its advantage lies.

I claim:

1. The method of controlling the waveform delivered by a vacuum tube oscillator having grid and plate circuits which comprises transferring energy from said plate circuit to said grid circuit in degree and ratio to provide an induced grid voltage in excess of that required to provide maximum plate current, and varying the impedance of said plate circuit both internally and externallyof the tube by control of the phase and magnitude of said grid current.

2. The method of generating current waves of non-sinusoidal forms with a vacuum tube having an external impedance connected to the elements thereof which comprises using said tube effectively as a switch to permit or prevent the flow of cmrent therethrough, and controlling the amount of said flow externally of said tube, by varying the impedance external to the tube to maintain the rate ofincrease of said current substantially constant throughout a major portion of each cycle.

3. The method of generating current waves of non-sinusoidal forms'with a vacuum tube havaosaosa ing grid and plate circuits associated therewith which comprises transferring energy from said plate circuit to said grid circuit in phase and ratio such that small increases in current in said plate circuit will produce minimum tube imped- 5 ance therein and small decreases in said plate current will produce substantially complete cutoff, controlling the rate of change of said current externally of said tube, and utilizing the amount of said current as a factor in the control of the rate of change thereof by varying the impedance external to the tubeto maintain the rate of increase fof said current substantially constant throughout a major portion of each cycle.

4. The method of generating current waveforms having substantially constant rates of increase during a major portion of the cycle with a vacuum tube oscillator which includes the step of causing a sufliciently large current flow to the grid of the tube at the beginning of said portion of the cycle to create minimum tube impedance and transferring said flow to the plate of the tube as the cycle progresses.

5. An oscillator for providing currents of sawtooth wave form comprising an iron-core transformer comprising a grid coil and a plate coil of diflerent natural periods, a vacuum tube having a plate connected to said plate coil and a grid, a condenser connected in series with said grid and grid coil, said grid and plate coils being pro- I portioned and phased so that any increase of potential on said grid is cumulatively effective to produce maximum positive potential thereon, and

a resistor bridged across said condenser, the impedance of said condenser and resistor being sufficiently low to provide an inappreciable bias to said grid except when a large proportion of the space current of said tube is flowing thereto.

6. An oscillator for producing currents of sawtooth wave form comprising a vacuum tube including cathode, anode .and control electrodes, a. predominately inductive plate circuit connecting said anode and cathode, a predominately inductive output circuit coupled-to said plate circuit, means for impressing potential changes on said control electrode of opposite sign from changes occurring on the anode and of such magnitude that any change of potential of the one is cumulatively effective to produce maximum opposite change of potential on the other, a conr denser of low impedance to the desired frequency of oscillation connected in series with said control electrode, and a resistor bridging said condenser and having an impedance not exceeding one order of magnitude greater than that of said condenser.

PHILO 'r. FARNSWORTH. l 

